The growing spread of cellular phones in recent years shows the demand for low cost transceivers. As for the phones, a wireless transmission system is planned to replace connection wires in other technical fields.
One conventional system to replace connection wires is the Bluetooth™ system. This is a low power, short range, and low cost radio communication system that is planned to replace wires or cables between computers, components, e.g. monitors and the like. The Bluetooth™ system operates at a frequency of 2.4 GHz. The frequency band is called ISM (Industrial Scientific and Medicine) and is destined for unlicensed low power radio operation up to a maximum irradiated power of 100 mW, or 20 dbm. With an irradiated power up to 100 mW a spatial transmission range of a few centimeters up to a few hundred meters can be attained. Due to limitations in different countries, a varying number of 23 to 79 channels having a bandwidth of 1 MHz are allocated for the unlicensed use. The Bluetooth™ system uses a frequency hopping algorithm to reduce interference caused by other users of the ISM band. The frequency hopping is executed at a rate of 1600 hops per second.
With such a short distance radio transmission all connection problems associated with e.g. laptop computers having periphery devices such as printers, storage means, or a network are easily solved, thereby obviating the need for a wire connection.
In most transceivers having variable frequency a PLL is used to generate the channel frequency. The use of a PLL is a well known and cheap way to build a variable frequency generator having almost the frequency stability of a quartz oscillator.
A standard PLL comprises at least one VCO, a frequency divider, a reference oscillator, a phase detector and a charge pump. In the standard operation, the VCO is oscillating at a frequency determined by the DC voltage provided by the charge pump, wherein the output oscillation from the VCO is divided by a frequency divider and the phases are compared with the phases of a reference oscillator in a phase detector, its output voltage determining the action of the charge pump, wherein the charge pump is controlling the DC voltage for determining an oscillation frequency of the VCO.
The output frequency of the PLL is determined by the division factor of the frequency divider and the frequency of the reference oscillator.
The PLL has one major restriction, in that its frequency range is limited by the resonance frequency range of the VCO. When using a Bluetooth™ module in countries with a frequency range of the ISM band limited to 23 channels, it is sufficient to use one VCO whose frequency range covers the 23 channels. The receiving and transmitting frequencies of the VCO are tuned in accordance with the frequency hopping algorithm.
It is important to know whether the VCO is operating linearly, to prevent energy from being wasted and the settling time frame being increased. So it is helpful to have an indicator to detect whether the PLL is in a linear operation state. It is state of the art to measure the DC input voltage of the VCO and check the output signal for its linearity to determine a relation between the input voltage and the linearity of the operation of the PLL. This includes that the input voltage of the VCO is slightly changed when it is measured.
In a PLL comprising a plurality of VCOs with overlapping frequency ranges, the PLL is designed such that one VCO is used in the not overlapping regions of the frequency range. For frequencies in the region of the overlap of two VCOs the PLL can alternatively be operated with one of both VCOs. In the overlapping region of the frequency ranges the problem resides in determining which of both VCOs should be used to generate a given frequency. It is therefore necessary to determine a boundary of the frequency range for each VCO. The VCO frequency range and its centre frequency (average) can depend on product tolerances. The boundary frequency must therefore be determined separately for each device.
The control voltage of the VCO can be used to determine if a VCO other than the operating VCO should be switched into the PLL to reach maximum linearity of the PLL output signal.
The main disadvantage of this known technique is the fact that the frequency range of each VCO has to be measured, the switching points have to be determined, and all these data have to be stored in a special storing means in the transceiver. The VCOs may in addition change their frequency ranges due to temperature shift and ageing effects.